JavaScript Array.indexOf() Method

1. Syntax
2. Parameters
3. Returns
4. Example Codes

We use the array.indexOf() method to find the first occurrence of the element in the reference array. Then, it is used to find the occurrence of the element in the given array.

Syntax

let values = ["Delft","Stack","Netherlands"];
let index = values.indexOf("element");
index = values.indexOf("element","startIndex");

Parameters

Returns

The Array.indexOf() returns -1 if the element is not found in the array; otherwise, it returns an index of the first occurrence of the element in the array.

Example Codes

Let’s learn the uses of the Array.indexOf() method by going through different code examples below.

Use Array.indexOf() to Find the Element in an Array

When we don’t pass the startIndex parameter to the Array.indexOf() method, it starts searching for the element from the 0th index.

The following code takes two different examples and finds the first occurrence of the element using the Array.indexOf() method. The output for the following code shows that for value Nether, the method returns the -1 as it is not in the array.

let values = ["Delft","Stack","Netherlands"];

let index1 = values.indexOf("Delft");
let index2 = values.indexOf("Nether");

console.log(index1)
console.log(index2);

Output:

0
-1

Use Array.indexOf() With Positive & Negative startIndex

When we add the positive startIndex, the method starts searching for the element from the startIndex and searches till the last index of the array. When the user passes the negative startIndex to array.indexOf(), it searches in the last startIndex number of elements from front to end.

For example, the following code fence has an array of length 5, and we have passed the -2 as a startIndex, so the method will search for the last 2 elements from the front to back.

let array = ["10.2","30","43.2","21","0"];

let index1 = array.indexOf("10.2",2);
let index2 = array.indexOf("21",-2);

console.log(index1)
console.log(index2);

Output:

-1
3

The Array.indexOf() method compares the element using the strict equality (===) operator, which means it also compares the element type. Furthermore, it performs the case-sensitive comparison for the string values.